The interest in pyrrole polymers derives from their stability and expected affordability compared to other electrically conducting polymers. The fragility and unprocessability of polypyrrole ("PP") films are drawbacks which have prevented the otherwise desirable conductive properties of PP films from being exploited. These drawbacks instigated the concept of combining PP with another polymer ("host") to form a composite which might provide the desirable physical properties of the host polymer and the conductive properties of the PP.
To this general end, the work done to date has been based on the well known facility with which pyrrole (PY) or thiophenes are electrochemically polymerized on conductive electrode surfaces, forming conductive films. The concept was to coat a metal or conductive glass electrode with a thin layer of a polymer which is normally an insulator (conductivity less than about 10.sup.-6 ohm.sup.-1 cm.sup.-1 or "S/cm"), then electrochemically deposit (electrodeposit) PP on the coated electrode.
In a specific illustration, a Pt electrode is coated with poly(vinyl chloride) "PVC" film 4-17 microns thick from a 10 g/liter tetrahydrofuran (THF) solution using low molecular weight, unstabilized commercial PVC. The composite film was prepared in a three-electrode, single compartment cell containing 0.0006 M pyrrole in 0.1 M tetraethylammonium fluoroborate-acetonitrile (dry) solution. If the PVC film on the electrode is thin enough, and sufficient voltage is used, PP is grown, first on the surface of the PVC film next to the electrode (inner surface), then, given enough time, through the film and on the surface of the PVC film away from the electrode (outer surface). The acetonitrile solvent was expected to swell the PVC and expose the Pt surface providing the necessary electrical path for formation of the PP containing BF.sub.4.sup.- ions. See "Conductive Composites from Poly(vinylchloride) and Polypyrrole" by DePaoli, M. et al. J. Chem. Soc., Chem. Commun., 1015-16, 1984. This work essentially corroborated the findings set forth in an article titled "Electrochemical Polymerization of Pyrrole on Polymercoated Electrodes" by Osamu Niwa et al. in J. Chem. Soc., Chem. Commun., 817-8, 1984. Niwa et al grew PP on an electrode coated with a 1.2 micron thick PVC film polymerized in acetonitrile solution containing tetraethylammonium perchlorate, so that the PP contained C10.sub.4.sup.- ions.
That the concentration of the electrolyte was a factor, and that the ions are retained in the PP film deposited was confirmed in an article titled "Conductive Composites prepared by Eletrochemically Polymerizing Pyrrole in Poly(Vinyl Chloride) blended with an Electrolyte" by Wang, T. T. et al J. Chem. Soc., Chem. Commun., 1343-44, 1985.
The only teaching relating to avoiding the direct use of a coated metal electrode on which a PP film can be deposited, is that of the electrochemical deposition of PP onto a polyacetylene anode which is sufficiently conductive to be substituted for a metal electrode. The morphology of the resulting composite depends on the initial doping state of the polyacetylene. Each of the components of the composite is conductive, and so is the composite. See "Composites of Conducting Polymers: PolyacetylenePolypyrrole" by Krische, Bernd et al., Mol. Cryst. Liq. Cryst. 121(1-4), 325-8, 1985. Since there is no apparent reason for depositing semiconductive PP on polyacetylene which is acknowledged to be relatively highly conductive compared with known organic polymers and is therefore a conductor, the morphology of the composite containing a single charged species of PP is of little consequence.
By "conductor" we specifically refer to a material which has a conductivity in excess of about 1 S/cm, recognizing that an organic material having a conductivity lower than 1 S/cm is usually deemed to merit a classification as a "semiconductor". Though each of the foregoing references were directed to providing a composite host/PP conductor, none addressed the problem of providing a composite of sufficient thickness which might be of practical use. For example, how is a PVC/PP composite a few microns thick to be used as a practical resistance heating element such as an aircraft de-icer, or a heating element for a seat of a frigid automobile?
It is known that when PP produced by a chemical oxidative polymerization is deposited in the pores of paper, the paper can be made semiconductive. In particular, German (FDR) Offenlegungsschrift DE No. 3321281 Al published Dec. 22, 1983 discloses a chemical process for producing a semiconductive paper by impregnating the paper with different concentrations of an aqueous ferric chloride solution which is acidified with HCl, then exposing the impregnated paper to PY monomer, usually in the gaseous phase. Further details of this process are disclosed in an article titled "Some Properties of Polypyrrole-Paper Composites" by Bjorklund, R.B. et al., Journal of Electronic Materials, Vol 13, No. 1, 1984.
Having formed the paper/PP composite they were interested in, it did not occur to them that such a semiconductive composite might possess high enough conductivity to be used as an electrode on which a different species of PP could be electrodeposited. It will be appreciated that chemically oxidatively polymerized PP (formed by oxidation with the halide of a Group VIII metal such as ferric chloride, say) contains FeCl.sub.4.sup.- counterion; electrodeposited PP, on the other hand, contains HSO.sub.4.sup.-, or BF.sub.4.sup.-, C10.sub.4.sup.-, and the like.
Moreover, though the impregnation of paper with PY monomer was practical because of a choice of paper having adequate porosity, there was no suggestion that either paper or an insulating synthetic resinous material ("host polymer") be swollen by PY monomer ("PY-swelled"), or by PY monomer in solution in a solvent capable of swelling the host polymer ("solvent-swelled"), to provide access for a suitable initiator for the PY which would then be oxidatively polymerized; and, having thus formed a semiconductive composite of host polymer and a chemically deposited ("electroless") species of PP, to use the semiconductive composite in a subsequent electrodeposition process.
It was in this technological framework that it was decided to produce a conductive composite of a normally insulating host polymer having an arbitrary shape such as a sheet, tube or rod, which shape was to be preserved after it was transformed into the conductive composite by depositing on, and in, the polymer a sufficient amount of each of two species of PP to make the composite a conductor. By "conductive composite" we refer to a composite which has a surface resistivity less than 50 ohms/square. For numerous applications, there was, and still is, a need for a "host polymer/PP composite", and particularly a synthetic resinous host/PP composite, which has sufficient stability under ambient conditions to be used as a heating element, or for EMI shielding, inter alia. Our invention fulfills this need.